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    <div class="sect1" lang="en" xml:lang="en">
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            <h2 class="title" style="clear: both"><a id="joins"></a>Database Joins</h2>
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      <p>
        If you have two or more secondary databases associated with a primary
        database, then you can retrieve primary records based on the intersection of 
        multiple secondary entries. You do this using a
        
        <span>join cursor.</span>
    </p>
      <p>
        Throughout this document we have presented a 
            
            <span>structure</span>
        that stores 
            
        information on grocery
            
            <span>vendors.</span>
        That 
            
            <span>structure</span>
        is fairly simple with a limited
        number of data members, few of which would be interesting from a query
        perspective. But suppose, instead, that we were storing 
        information on something with many more characteristics that can be queried, such
        as an automobile. In that case, you may be storing information such as
        color, number of doors, fuel mileage, automobile type, number of
        passengers, make, model, and year, to name just a few.
    </p>
      <p>
        In this case, you would still likely be using some unique value to key your
        primary entries (in the United States, the automobile's VIN would be
        ideal for this purpose). You would then create a 
            
            <span>structure</span>
        that identifies
        all the characteristics of the automobiles in your inventory. 
        
        
    </p>
      <p>
        To query this data, you might then create multiple secondary databases,
        one for each of the characteristics that you want to query. For
        example, you might create a secondary for color, another for number of
        doors, another for number of passengers, and so forth. Of course, you
        will need a unique 
            
            <span>key extractor function</span>
        for each such secondary database. You do
        all of this using the concepts and techniques described throughout this
        chapter.
    </p>
      <p>
        Once you have created this primary database and all interesting
        secondaries, what you have is the ability to retrieve automobile records
        based on a single characteristic. You can, for example, find all the
        automobiles that are red. Or you can find all the automobiles that have
        four doors. Or all the automobiles that are minivans. 
    </p>
      <p>
        The next most natural step, then, is to form compound queries, or joins.
        For example, you might want to find all the automobiles that are red,
        and that were built by Toyota, and that are minivans. You can do this
        using a 
            
            <span>join cursor.</span>
    </p>
      <div class="sect2" lang="en" xml:lang="en">
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            <div>
              <h3 class="title"><a id="joinUsage"></a>Using Join Cursors</h3>
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        <p>
            To use a join cursor:
        </p>
        <div class="itemizedlist">
          <ul type="disc">
            <li>
              <p>
                    Open two or more 
                        
                    cursors 
                        
                        <span>for</span> 
                    secondary databases that are associated with
                    the same primary database.
                </p>
            </li>
            <li>
              <p>
                    Position each such cursor to the secondary key
                    value in which you are interested. For example, to build on
                    the previous description, the cursor for the color
                    database is positioned to the <tt class="literal">red</tt> records
                    while the cursor for the model database is positioned to the
                    <tt class="literal">minivan</tt> records, and the cursor for the
                    make database is positioned to <tt class="literal">Toyota</tt>.
                </p>
            </li>
            <li>
              <p>

                    Create an array of  cursors, and
                    place in it each of the cursors that are participating in your join query. 
                    <span>Note that this array must be null terminated.</span>

                </p>
            </li>
            <li>
              <p>

                    Obtain a join cursor. You do this using the 
                         
                         
                        <tt class="methodname">Db::join()</tt> 
                    method.  You must pass this method the array of secondary cursors that you
                    opened and positioned in the previous steps.
                </p>
            </li>
            <li>
              <p>
                    Iterate over the set of matching records
                        
                    until
                    
                    <span>the return code is not <tt class="literal">0</tt>.</span>
                </p>
            </li>
            <li>
              <p>
                    Close your  cursor.
                </p>
            </li>
            <li>
              <p>
                    If you are done with them, close all your  cursors.
                </p>
            </li>
          </ul>
        </div>
        <p>
            For example:
        </p>
        <a id="cxx_index9"></a>
        <pre class="programlisting">#include &lt;db_cxx.h&gt;
#include &lt;string.h&gt;

...

// Exception handling omitted

int ret;

Db automotiveDB(NULL, 0);
Db automotiveColorDB(NULL, 0);
Db automotiveMakeDB(NULL, 0);
Db automotiveTypeDB(NULL, 0);

// Database and secondary database opens omitted for brevity.
// Assume a primary database:
//   automotiveDB
// Assume 3 secondary databases:
//   automotiveColorDB  -- secondary database based on automobile color
//   automotiveMakeDB  -- secondary database based on the manufacturer
//   automotiveTypeDB  -- secondary database based on automobile type

// Position the cursors
Dbc *color_curs;
automotiveColorDB.cursor(NULL, &amp;color_curs, 0);
char *the_color = "red";
Dbt key(the_color, strlen(the_color) + 1);
Dbt data;
if ((ret = color_curs-&gt;get(&amp;key, &amp;data, DB_SET)) != 0) {
    // Error handling goes here
}

Dbc *make_curs;
automotiveMakeDB.cursor(NULL, &amp;make_curs, 0);
char *the_make = "Toyota";
key.set_data(the_make);
key.set_size(strlen(the_make) + 1);
if ((ret = make_curs-&gt;get(&amp;key, &amp;data, DB_SET)) != 0) {
    // Error handling goes here
}

Dbc *type_curs; 
automotiveTypeDB.cursor(NULL, &amp;type_curs, 0);
char *the_type = "minivan";
key.set_data(the_type);
key.set_size(strlen(the_type) + 1);
if ((ret = type_curs-&gt;get(&amp;key, &amp;data, DB_SET)) != 0) {
    // Error handling goes here
}

// Set up the cursor array
Dbc *carray[4];
carray[0] = color_curs;
carray[1] = make_curs;
carray[2] = type_curs;
carray[3] = NULL;

// Create the join
Dbc *join_curs;
if ((ret = automotiveDB.join(carray, &amp;join_curs, 0)) != 0) {
    // Error handling goes here
}

// Iterate using the join cursor
while ((ret = join_curs-&gt;get(&amp;key, &amp;data, 0)) == 0) {
    // Do interesting things with the key and data
}

// If we exited the loop because we ran out of records,
// then it has completed successfully.
if (ret == DB_NOTFOUND) {
     // Close all our cursors and databases as is appropriate,  and 
     // then exit with a normal exit status (0). 
} </pre>
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